In the face of a significant and unusual deployment of Russian troops on Ukraine's borders, NATO Secretary-General Jens Stoltenberg warned Russia that the western military alliance was standing by Ukraine. 

Stoltenberg urged Russia to be transparent about military activity in order to calm tensions and prevent an escalation, emphasising that the most important thing right now was to keep things from spiralling out of control.

"We must be clear-eyed and realistic about the issues we face. And what we're seeing is a significant, large Russian military build-up" Stoltenberg said.

"We observe an unusual concentration of forces, and we know that Russia has been willing to deploy these types of military capabilities before to conduct aggressive measures against Ukraine," he said.

NATO - Opinion: Press conference by NATO Secretary General Jens Stoltenberg following the the meeting of the North Atlantic Council at the level of Defence Ministers, 08-Nov.-2017

Russian army movements in recent days have sparked worries of an impending attack. Moscow has dismissed such suggestions as inflammatory and complained about growing activity in the area by the NATO transatlantic alliance. 

Russia annexed Crimea from Ukraine in 2014 and claims ownership of the surrounding waters, despite the fact that other countries continue to recognise the peninsula as Ukrainian.

According to Stoltenberg, the troop border build-up, which Ukraine estimated at 100,000 last week, was dangerous since it lowered the amount of warning time if Russia decided to "perform a military aggressive move against Ukraine."

"Large equipment such as tanks, self-propelled artillery, and infantry fighting vehicles are moved at night to avoid tell-tale pictures showing up on social media as they did during the Russian military build-up in spring," a NATO source said when asked to describe how Russia is deploying its military equipment to Ukraine.